In recent years, for electronic devices which are required to be small and low-profile like portable terminals, techniques for fabricating multi-layer circuit boards capable of high-density packaging have been being developed. Important technical assignments to fabricate multi-layer circuit boards by a build-up process include: techniques for forming interlevel insulator layers to be laminated in multi-layers; techniques for forming interconnections; and techniques for forming connection conductors between the layers. Above all, it has been expected to develop an inexpensive method for forming a large number of via holes with high precision and filling these via holes with a conductive member.
Via hole formation methods developed include: using laser processing; and forming throughhole conductors by previously providing a surface with conductive projections which are to be via hole conductors, and then pressing softened resin onto the surface.
In recent years, other approaches have been developed. One such approach is to form through holes on prepregs by press working and to laminate the prepregs. Another is to form a resin sheet having through holes therein by pouring resin into a metal mold provided with projections for via hole formation.
To be more specific, Japanese Laid-Open Patent Application No. 2002-76614 discloses a method for forming through holes by heating a top mold having metal pins made to stand thereon at a prescribed pitch up to the melting point of a prepreg, and at the same time, heating the prepreg close to its melting point so as to make the metal pins pierce the prepreg; and raising the top mold after being cooled so as to pull out the metal pins.
Japanese Laid-Open Patent Application No. 2001-230520 discloses a method for forming a resin sheet having through holes therein by pouring molten resin into a metal mold provided with projections, which is what is called injection molding.
This conventional method for forming a resin sheet having through holes therein by injection molding will be described as follows with reference to accompanying drawings. FIGS. 9A to 9C are cross sectional views to depict processes in the conventional method for fabricating a resin sheet having through holes therein.
First of all, as shown in FIG. 9A, bottom mold 101 and top mold 102 are disposed in such a manner that second projections 104 of bottom mold 101 are in contact with top mold 102. Bottom mold 101 is provided with first projections 103 on sites corresponding to trenches in which to form conductive interconnections in a later process, and with second projections 104 on sites corresponding to through holes. Top mold 102 is designed to be flat to fit bottom mold 101. Top mold 102 and bottom mold 101 form a metal mold with cavities. To make a description simple, the cross sectional view shows main components only, while omitting surrounding barrier ribs for forming the mold cavities, a gate through which to pour resin and other components.
Next, as shown in FIG. 9B, resin 105 with thermoplastic or thermosetting properties is heated up to its melting point and poured into the cavities. After the pouring, the metal mold is cooled, and top mold 102 and bottom mold 101 are pulled out of resin 105 to obtain resin sheet 107 having a cross section shown in FIG. 9C. Resin sheet 107 is provided with trenches 108 at sites corresponding to first projections 103 of bottom mold 101, and with through holes 109 at sites corresponding to second projections 104. By filling conductive paste into trenches 108 and through holes 109 and then hardening the paste, resin sheet 107 provided with an interconnection pattern including throughhole conductors can be obtained. Integrally laminating a plurality of resin sheets 107 provided with this interconnection pattern results in a multi-layer circuit board.
However, all conventional methods described above address throughhole formation on a board having no interconnection pattern. It is difficult to form an interlevel insulator film having through holes therein directly onto a board having an interconnection pattern formed therein.
The present invention, which has been contrived to solve the problem of the conventional methods, has an object of providing a circuit board fabrication method in which an interlevel insulator layer having via holes therein can be easily formed onto an insulator substrate having an interconnection pattern formed therein while securing fine pitch interlevel connection, and also providing such a circuit board.